This invention relates to phosphate laser glasses, typically neodymium-doped, having desirable cross section for stimulated emission for high energy use, high thermal conductivity and low thermal expansion, inter alia, in comparison to prior art and commercially available phosphate laser glasses. The invention especially relates to phosphate glass useful as a laser amplifier in a multi-pass, high energy laser system.
The term "laser" refers to the amplification of light by the stimulated emission of radiation. In a laser, a suitable active material, which can be a glass suitably doped with an active atomic species such as neodymium, is placed in a cavity resonator formed by two reflecting, or at least partially reflecting, walls.
Glasses for high average power use are required to withstand thermal heating due to high repetition rate, usually 5-10 Hz. These glasses also need high cross section for better efficiency.
On the other hand, high energy applications focus on the amount of energy stored in the amplifier to obtain very high energy (Tera Watts and higher). In such cases, factors which cause reduction in stored energy must be eliminated. These factors are ASE (amplified spontaneous emission) and PO (paracitic oscillation). The former becomes a serious problem when disc size increase; the effect increases as cross section increases. The latter is more a function of geometry. Thus, for high energy applications, glass must have a low cross section to reduce ASE, and high lifetime to maintain good efficiency despite its low cross section.
It is important for the application of large-aperture laser amplifiers that the active material be characterized by a low cross section for stimulated emission and long fluorescence lifetime of the excited state involved in the laser transition. In a large-aperture amplifier, in which the size of ASE becomes significant, a major effect is to reduce the fluorescence lifetime, which decreases the efficiency of pumping. Such a large ASE effect is accompanied with high cross section; therefore, it is necessary to have low cross section glass for this application in order to maintain high efficiency.
Good thermomechanical properties, such as low CTE and high thermal conductivity, lead to the ease of manufacturing laser glasses of good optical quality. These properties are advantageous for forming glass, annealing, processing, and also for general handling. These good thermomechanical properties also translate into a good thermomechanical figure of merit, FOM, given by: ##EQU1## where S is the fracture strength; K.sub.90.degree. C., the thermal conductivity; .upsilon., Poisson's ratio; E, Young's modulus; and .alpha., the thermal expansion coefficient of the material. This is an important material characteristic when material is employed as high average power active material. For a more detailed discussion of FOM, see U.S. Pat. No. 4,929,387.
Solid state laser materials are also more favorable for application in high energy laser systems if the active material can be produced in large sizes with high optical quality, high optical homogeneity, and in a form free of all inclusions which absorb radiation.
The inclusions could develop into damage sites within the material when the laser is operated at high power levels, leading ultimately to the active element being rendered useless as a laser material. It is always necessary that the glass have good manufacturability properties, e.g., devitrification stability.
It is known that phosphate laser glasses have a low threshold value for the laser effect; and phosphate compositions have been commercially available for some time as materials for use in laser systems in large sizes, with excellent optical quality and optical homogeneity. The quality of prior-art phosphate laser glasses recently has been extended by the introduction of new manufacturing technology capable of producing these compositions as glasses with levels of optical quality as good as that of previous glasses but which are now free of all absorbing inclusions which could develop into laser damage sites within the glass.
Nevertheless, a need has remained to further the development of phosphate compositions, e.g., achieve significantly lower emission cross section values in phosphate laser glasses, in order to reduce ASE, thus making available new compositions which are more attractive for use in high energy, large-aperture laser amplifier systems, while retaining the current state-of-the-art qualities which make doped phosphate glasses so useful as laser media.
Prior art phosphate glasses contain a wide variety of components including, for example, Al.sub.2 O.sub.3, SiO.sub.2, alkali metal oxides (Na.sub.2 O, K.sub.2 O, Li.sub.2 O, especially), alkaline earth metal oxides, etc., in addition to the base component P.sub.2 O.sub.5. The prior art glasses having the best combination of the important thermal properties of thermal conductivity and coefficient of thermal expansion have typically been those containing necessary amounts of SiO.sub.2. See, e.g., DE 3435133, JP 51-107312 and DE 3609247. These glasses typically have relatively low alumina contents.
Other phosphate laser glasses place no special emphasis on SiO.sub.2 or even lack it entirely, e.g., U.S. Pat. Nos. 3,846,142; 4,248,732; 4,075,120; 4,239,645; 4,022,707; and 4,470,922; JP 51-59911, 62-18495, and 63-233024; DE 2924684 and 3340968; etc.